Phospholipases are a class of ubiquitous enzymes that have in common their substrate and the fact that they are all esterases. Beyond that, they are a diverse group of enzymes that fall into two broad categories, the acyl hydro- lases and the phosphodiesterases. The former group is made up of the phos- pholipases Al and A , phospholipase B, and the lysophospholipases. On the 2 other hand, the phosphodiesterases are the phospholipases C and D. The scheme indicates the site of attack of each type of phospholipase. PLA 1 PLB~j ft 0\ ~-C-O-C-R d ~ 2 I 1 R-C-0-C-H 0 2 /H2-6-0-U-0-x PLA, ~ 6- '" PLC PLD The lysophospholipases, not shown, have in some cases properties similar to phospholipase B and are known to attack the acyl ester at either position 1 or position 2 of the glycerol backbone. Furthermore, some of the phos- pholipases C and D do not hydrolyze phosphoglycerides but use sphingo- myelinase as their substrate. These phospholipases C are also referred to as sphingomyelinases. The products of that reaction are phosphocholine plus ceramide.